Switched-capacitor (SC) DC-DC converters are comprised of capacitors and switches and accomplish power conversion solely by charging and discharging capacitors. Due to their lack of magnetic elements, SC converters are commonly used in integrated implementations which require higher power density.
Switched-capacitor DC-DC convertors are capable of attaining high step-up and high step-down conversion ratios, an important requirement in many applications, including 48V-to-1V power conversion in telecommunications and data-center applications. Converters with high step-up and step-down ratios can eliminate the need for intermediate power conversion stages and additional intermediate bus converters, allowing for direct conversion from source to load. This approach can improve overall conversion efficiency and power density and reduce system cost and size.
The ideal, unloaded conversion ratio, Mi, of a switched-capacitor DC-DC converter is entirely determined by its switching structure. The maximum and minimum attainable ideal conversion ratio of a switched-capacitor converter has been shown to be a function of the number of capacitors. SC converters using an increased number of capacitors are therefore capable of attaining higher step-down and higher step-up conversion ratios.
While a large number of active devices can be integrated in a small area using modern CMOS processes, current integrated capacitors typically achieve densities of 0.1-10 nF/mm2. This low capacitive density significantly increases the size of the converter and thus its cost. In addition, integrated capacitors exhibit high parasitic bottom-plate capacitances, degrading the efficiency and performance. Accordingly, the number of capacitors used by the converter should be minimized to the greatest extent possible.
A number of switched-capacitor DC-DC converters with high step-up and high step-down conversion ratios have been proposed. These include those found in U.S. Pat. No. 6,198,645 issued to National Semiconductor Corporation on Mar. 6, 2001, U.S. Pat. No. 8,259,476 issued to Ben-Yaakov et al on Jul. 29, 2009, U.S. Pat. No. 8,693,224 issued to pSemi Corporation on Apr. 8, 2014, and U.S. Pat. No. 8,817,501 issued to pSemi Corporation on Aug. 26, 2014. These prior converters, however, cannot generate ideal conversion ratios greater than Mi=2N or less than Mi=1/2N, where N is the number of capacitors used by the converter.